Production of amino acids



Patented Oct. 24, 1950 2,527,366 Pnonuo'rIoN F AMINO ACIDS John E. Livak. Clemson, S. 0., and Edgar C. Britton, Midland, Mich., assignors to The Dow Chemical Company, Midland, Mich., a corporation of Delaware No Drawing. Application November 7, 1946, Serial No. 708,469

18 Claims. (01. 260-319) This invention concerns an improved method for the production of alpha-amino monocarboxylic acids from corresponding hydantoins.

The hydantoins employed as starting materials in the method of this invention have the general formula:

to hydrolysis, e. g. by heating the same together with an aqueous alkali solution, react slowly with formation of a series of hydrolysis and decomposition products including a corresponding hydantoic acid as an initial product and an alphaamino monocarboxylic acid as one of the subsequent products. Such hydrolyses have heretofore been carried out by heating, at atmospheric pressure, a hydantoin together with an aqueous solution of a hydrolyzing agent such as sulphuric acid, hydrochloric acid, hydrobromic acid, barium hydroxide, or potassium hydroxide, etc. Because of the long periods of heating, particularly when using alkaline hydrolyzing agents, required to form a substantial amount of an alpha-amino monocarboxylic acid, the low yields of the latter, and difficulties encountered in purifying the crude product, such hydrolysis, as heretofore carried out, is not satisfactory as a step in the manufacture of alpha-amino acids.

It is an object of this invention to provide an improved method for the production of alphaamino monocarboxylic acids from hydantoins which permits rapid hydrolysis of a hydantoin to form a corresponding alpha-amino monocarboxylic acid in good yield and which permits ready recovery of the amino acid; or a salt thereof, in a form of high purity. Other objects will be apparent from the following description of the invention.

We have found that a hydantoin may be hydrolyzed rapidly to produce a corresponding alpha-amino monocarboxylic acid in excellent yield by heating the same together with aqueous barium hydroxide under pressure, e. g. within a bomb or autoclave, to temperatures above C. and usually within the range of from 115 to 210 C. It may be mentioned that these conditions of temperature and pressure are not satisfactory for alkaline hydrolyzing agents in general. For instance, ammonium hydroxide and lime, when tested as hydrolyzing agents for hydantoins under similar conditions of temperature and pressure, either caused excessive byproduct formation and produced the alpha-amino monocarboxylic acid in an unsatisfactorily low yield or resulted in formation of a badly discolored amino acid product which could not satisfactorily be decolorized. Also, barium hydroxide itself, when tested as a hydrolyzing agent at atmospheric pressure, was unsatisfactory, i. e. upon boiling a mixture of a hydantoin and aqueous barium hydroxide at atmospheric pressure, hy-

drolysis occurred very slowly and the yield of alpha-amino monocarboxylic acid was low.

The products formed by the hydrolysis of a hydantoin with aqueous barium hydroxide under the conditions of the invention comprise insoluble barium carbonate, ammonia, and a watersoluble barium salt of an alpha-amino monocarboxylic acid. The latter may, if desired, be separated by filtering the mixture and evaporating the liquor to crystallize the barium salt, but since barium salts are toxic it is important, for medicinal uses, that the amino acid product be recovered either in free form or as a substantially non-toxic salt thereof.

We have further found that an alpha-amino monocarboxylic acid, present as a soluble barium salt in such hydrolysis mixture, may be recovered in free form, or as an alkali metal salt thereof, by treating the hydrolysis mixture with an agent capable of precipitating the barium as an insoluble barium salt, removing the precipitate, and concentrating the liquor to crystallize the amino acid compound therefrom. Examples of precipitating agents which may be employed for this purpose are sulphuric acid, sodium sulphate, sodium acid sulphate, potassium sulphate, ammonium sulphate, ammonium acid sulphate, carbon dioxide, sodium carbonate, sodium bicarbonate, potassium carbonate, potassium bicarbonate, ammonium carbonate, or ammonium bicarbonate, etc. Use of an alkali metal sulphate or carbonate as the agent for precipitating the barium results in formation of a corresponding watersoluble alkali metal salt of the amino acid product. The precipitated barium sulphate or caraccuses db bonate may be removed. e. g. by filtration, and the l quor concentrated by evaporation to crystallize and recover the alkali metal salt of the amino acid. If desired, the latter may be acidifled to obtain the amino acid in free form.

Use ofsulohuric acid or carbon dioxide as the agent for precipitating the barium from the hydrolysis mixture results in direct liberation of the amino acid in free form. In most instances the alpha-amino monocarboxylic acids are quite soluble and remain dissolved in the liquor from which a barium salt is precipitated and removed. An exception is the amino acid, tryptophane, which, in free form, is only moderately solublein water and tends to precipitate together with the insoluble barium salt. Accordin ly. in the production of tryptophane by hydrolysis of 5-(3-indolylmethyl) hydantoin with aqueo s barium hydroxide, we prefer subsequently to preci itate the barium by addition of an alkali metal sulphate or carbonate so as to leave the trjvptophane dissolved as an alkali metal salt thereof? The resultant insoluble barium salt may be removed. after which the liquor may either be evaporated to crystallize the alkali metal salt of tryptophane or may be neutralized, e. g. to a pH value of from 5 to 8 and preferably from '1 to 8 with HCl, HBr, H2804. or acetic acid etc.,

to precipitate and recover the tryptophane in free form.

Use of ammonium sul hate. ammonium acid sul hate ammonium carbonate. or ammonium acid carbonate as the agent for precipitating barium from the hydrolysis mixture results in formation of an ammonium salt of the amino acid product. but the ammonium salt under oes dissociation to evolve ammonia and form the free amino acid pon subseouent heating, e. g. during evaporation of the liquor to crystallize the product. Accordingly. the use of sulphuric acid, carbon dioxide, or the above-mentioned ammonium salts as agents for the. precipitation of barium from a hydrolysis mixture leads ultimately to formation and recovery of the amino acid product in free form. In most instances the free amino acid products are water-soluble and. when formed, remain dissolved in the liq or from which the barium is precipitated, in which case carbon dioxide or ammonium carbonate is preferably used as the agent for precipitation of the barium.

In instances in which an al ha-amino monocarboxylic acid product formed by hy rol sis of a hydantoin crystallizes from the hydrolysis liquor in a discolored, or impure, form, we have found that it may readily be purified merely by washing or digesting the same with a lower aliphatic monohydric alcohol. e. 8. methyl. ethyl, propyl. isopropyl. or butyl alcohol, etc., in amount sufficient to form a slurry of the amino acid and filtering the mixture. Usually. a minor amount of the amino acid is dissolved by the alcohol and may be crystallized from the washings. However, the alcohols serve as preferential solvents for by-products formed in the hydrolysis of a hydantoin and such washing with alcohol of the, crystalline alpha-amino monocarboxylic acid product permits recovery of the product in substantially pure form.

In hydrolyzing a hydantoin by the method of this invention, a bomb or autoclave is charged with .the hydantoin and an aqueous solution or slurry of barium hydroxide. Approximately 1.5 molecular equivalents or more, e. g. from 1.5 to

d 3 moles, of barium hydroxide are preferably employed per mole of the hydantoin, but the barium hydroxide can be used in somewhat smaller amount or in as large a proportion as desired. Water is preferably present in amount exceeding that necessary to dissolve the barium hydroxide, but this is not required. It is necessary,

of course, that water be present in amount exceeding that theoretically required for hydrolysis of the hydantoin. Prior to heating the mixture to a reaction temperature, the reactor is preferably swept free of air, e. g. with steam, nitrogen, or other inert gas, since oxygen, if present during the hydrolysis, may cause by-product formation.

The mixtureis heated under pressure in they bomb or autoclave at temperatures above 115 C., and preferably within the range of from 115 to 210 C., for a time suflicient to effect conversion of a major portion of the hydantoin to a. corresponding alpha-amino monocarboxylic acid. The-time required for the hydrolysis is usually within the range of 5 minutes or less at 210 C. to 10 hoursor thereabout at 115 C., but the time varies also, of course, with change in the kind of hydantoin subjected to the hydrolysis. In gen ral, it is preferable that the mixture be cooled to 100 C., or lower, as quickly as possible after completing the hydrolysis reaction. Completion of the hydrolysis reaction may be ascertained by determining the ammonia content of the hydrolysis mixture.

After com leting the hydrolysis, the mixture is cooled to 100 C. or less. preferably to about room temperature, and the reaction vessel is opened.

The mixture is treated with an agent capable of causing substantially complete precipitation of the barium, e. g. as barium sulphate or carbonate, to leave the amino acid, or a soluble salt thereof, dissolved in the liquor. In instances in which an alkali metal salt of the amino acid is desired as the product, or when an amino acid Ksuch as tryptophane, which in free form is only sparingly soluble in water is formed, an alkali metal sulphate or carbonate is preferably added to precipitate a corresponding salt of barium which is removed. e. g. by filtration, leaving an alkali metal salt of the amino acid dissolved in the liquor. When the liquor contains an alkali metal salt of tryptophane, it may, if desired, be neutralized, e. g. with HCl, HBr, H2804, or acetic acid, etc., to precipitate the tryptophane which may be separated. Alternatively. a liquor freed of barium, but containing an alkali metal salt of an amino acid product may be concentrated by evaporation to crystallize the salt and the latter be separated. Sodium and potassium salts of amino acids are readily obtained by this procedure.

When, as is usually the case, the amino acid formed by the hydrolysis is quite soluble in water and is to be recovered in free form as the crystalline compound, the agent employed to precipitate barium from the hydrolysis mixture may be sulphuric acid, ammonium sulphate, ammo- I nium acid sulphate, carbon dioxide, ammonium wise with alternate cooling to crystallize and separate successive crops of the amino acid. When carried out in such stepwise manner, the first several crops of crystalline amino acid, comprising a major amount of the product, after being washed free of adhering mother liquor are usually of high purity. Any discolored or impure amino acid obtained during such evaporation and crystallization operations may be purified by admixing therewith a lower alcohol, preferably ethyl alcohol, in amount sufficient to form a slurry and filtering or draining the alcohol from the undissolved amino acid. The alcoholic washings may then be evaporated to crystallize therefrom the minor amount of amino acid dissolved in the washing operation. The amino acid thus recovered from the washings usually is substantially pure.

The following examples describe certain waysin which the principle of the invention has been employed, but are not to be construed as limiting its scope.

EXAMPLE 1 In each of three experiments, S-(beta-methylmercaptcethyl) hydantoin was hydrolyzed by heating the same together with an aqueous solution of an alkali in a bomb at temperatures of from 155 to 160 C. The experiments difiered from one another with regard to the kind and proportion of alkali used and the time of heating, as indicated in the following table. After completing each hydrolysis, the yield of the amino 'acid, dl-methionine, was determined. Table I names the alkali employed in each experiment, gives the gram molecular equivalents both of said alkali and of the 5-(beta-methylmercaptoethyl) hydantoin and the grams of water initially present in each reaction mixture. It also gives the time over which each mixture was heated at the above-mentioned reaction temperature and the per cent yield of methionine, based on the 5- (betamethylmercaptoethyl) hydantoin starting mate- The methionine isolated as a pure white crystalline compound by treating the hydrolysis mixture with ammonium carbonate, removing the resultant precipitate of barium carbonate by filtration, evaporating the filtrate to dryness under vacuum, and washing the residual methionine product with ethyl alcohol. When the hydrolysis mixture of run 2 was similarly treated, methionine of good purity, but noticeably discolored, was obtained. From this series of. experiments it will be seen that barium hydroxide, under suitable reaction conditions, is far superior to ammonium hydroxide or lime as an agent for the hydrolysis of a hydantoin to produce a corresponding alphaamino monocarboxylic acid.

EXANIPLE 2 obtained in run 3 was readily -millimeters absolute pressure.

ous barium hydroxide as an agent for the hydrolysis of a hydantoin at the atmospheric boiling temperature of a hydrolysis mixture and at the somewhat higher reaction temperatures required by the method of this invention. In one experiment, a mixture of 0.015 gram mole of 5- (beta-methylmercaptoethyl) hydantoin, 0.1 mole of barium hydroxide octahydrate, and 45 grams water was heated to boiling under reflux for 5 hours, the temperature of the mixture being from 103 to 105 C. during this period. Thereafter, the mixture was cooled, treated with ammonium carbonate in amount sufl'lcient to precipitate the barium as barium carbonate and filtered. The filtrate was evaporated to dryness by boiling the same under vacuum, i.-e. at approximately The residue was washed thoroughly with ethyl alcohol, leaving substantially pure dl-methionine as the undissolved material. The washings were evaporated to concentrate the same and --a further amount of methionine was crystallized therefrom. The total yield of methionine was 15.6 percent of theoretical, based on the 5-(beta-methylmercaptoethyl) hydantoin starting material.

In each of the other experiments 5-(betamethylmercaptoethyl) hydantoin was hydrolyzed by heating the same together with an aqueous barium hydroxide solution in a bomb at the temperatures and for the times given in Table II. Each bomb was then cooled, opened, and the hydrolysis mixture was treated with sufficient ammonium carbonate to precipitate the barium as barium carbonate. The latter was removed by filtration. Each filtrate was evaporated by heating the same to boiling at approximately 150 millimeters absolute pressure until a substan tially dry residue of the crude methionine product was obtained, The residue was washed with about 4 times its weight of ethyl alcohol to obtain substantially pure, white, crystalline dimethionine as the undissolved material. The washings were concentrated by evaporation and a further minor amount of methionine was crystallized therefrom. Table II gives the gram molecular equivalents of barium hydroxide and of 5 (beta methylmercaptoethyl) hydantoin and, also, the grams of water used in each experiment; the reaction temperature to which the mixture was heated under pressure; and the time over which it was maintained at such reaction temperature. It also gives the per cent yield of dl-methionine obtained in each experiment, based on the 5-(beta-methylmercaptoethyl) hydantoin startin material. It may be mentioned that, in the experiment wherein a reaction mixture was heated to a temperature of 200-205 C., the mixture was cooled immediately after being brought to said temperature. In this instance, the time of heating is given as less than one minute.

accuse The purpose of this example is to present a number of instances in which the method the invention has been applied in hydrolyzing van.

8 sultant alpha-amino monocarboxylic acid compound dissolved in the liquor, removing the precipitatqprystallizing the alpha-amino monocarboxylic acid compound from the liquor, and washous hydantoins to produce corresponding alpha- 6 8 y impure crystalline i -lphw mino monoamino monoc'arboxylic acids. In each experifl boxylic acid P d w h i l q id 11!- ment, a reaction mixture having the composition e liphatic alcohol to remove the impurities. given in Table III was heated with agitation in In method wherein hydanteln 18 y r a bomb at the temperature and for the time lyzedtoi'ormacorrespo l alpha-amino monogiven. Thereafter the bomb was cooled, opened, 10 te. the eps which consist in accomand the hydrolysis mixture was treated with amh s h hydroly is of the hyd n n y heatmonium carbonate to precipitate the barium con- 2 011% lar equivalent of the same to h r in; thereof as barium carbonate The g gwith an aqueous solution 0! at least 1.5 moles oi tate was removed by filtration and the filtrate bl-rium hydroxide at an u o fi lllpemtmolwas evaporated under vacuum substantially to 15 Dherle p r mperatures between 115 and dryness. The residual impure alpha-amino o fter pr c pitating the barium as a monocarboxylic acid was washed thoroughly water-insoluble barium salt to leave the resultant with alcohol, whereby it was obtained in substanlpha-amino monocarboxylic acid compound distially pure form. In most instances, the wash- -801Ved in the quor, removing the precipitate and ings were concentrated by evaporation and a crys allizing the alpha-amino monocarboxyiic further amount of amino acid was crystallized eid compound from the liquor. therefrom. Table III names the hydantoin deriv- 3. In a method of making alpha-amino monoative subjected to hydrolysis ineach experiment carboxylic acids and salts thereof, the steps or and gives the gram moles thereof, the gram moles hydrolyzing a hydantoin having the general for. of barium hydroxide used, as a hydrolyzing mula: agent. and the grams 0! water contained in the hydrolysis mixture. It also gives the temperature to which each hydrolysis mixture was heato= =0 ed under pressure and the time of heating. The table names the alpha-amino monocarboxylic acid obtained in each experiment and gives the wherein R, represents a non-reactive monovalent per cent yield thereof, based upon the hydantoin organic radical having a carbon atom thereof atderivative used as a starting material. tached to the hydantoin nucleus, by heating, at an Table III Reaction Mixture igg f Product Rim No. Hydauwin Mom,

Gm H3O Temp. 7 Time Kind Yield Kind Moles sac-1H4 and 40: Gram C'. Minute: p cm 1 S-ethylhydanioin 0.1 0.l6 300 176-180 15 alpha-aminobutyric acld 71.8 b-iaogropyl hydanto'm. 16.5 27 40,000 140 90 111- 811118 87.5 5-150 utylhydantoin 0.015 0.12 250 110-175 30 dl-leucine. 93.5 5-(sec.-butyl) hydantoin 0.06 0.08 250 1 dl-isoleucine 9L3 5-benzyl hydanmin 0.06 018 300 175-180 20 dl-phenylalanine. 315 5-(3-indolyl-mothyl) hydantoi11 0.05 0.10 300 147-155 as dl-tryptophane 88 o 5-(4-hydroxy-3-methoxybenzyl)hy- 0.06 0.095 $0 160-155 40 be a-(i-hydroxy-s-metlioxyag danioin. phenyl) alanine.

Other hydantoins may be hydrolyzed by the autogenous superatmospheric pressure and in the method herein described to obtain corresponding alpha-amino monocarboxylic acids in good yields. For instance, 5,5-dimethylhydantoin may be hydrolyzed to obtain alpha-amino-isobutyric acid,

or 5-ethyl-5-methyl-hydantoin may be hydrolyzed to obtain alpha-amino-alpha-methyl-butyric acid, etc.

Other modes of applying the principle of the invention may be employed instead of those explained, change being made as regards the method or compounds herein-disclosed, provided the step or steps stated by any of the following claims or the equivalent of such stated step or steps be employed.

We therefore particularly point out and distinctly claim as our invention:

1. In a method wherein a hydantoin is hydrolyzed to form a corresponding alpha-amino monocarboxylate, the steps which consist in accomplishing the hydrolysis of the hydantoin by heating the same together with aqueous barium hydroxide at a superatmospheric pressure as great as-the autogenous pressure to temperatures above 115? C., thereafter precipitating the barium as substantial absence of air, one molecular equivalent of the hydantoin together with an aqueous solution of at least 1.5 molecular equivalents of barium hydroxide to temperatures between and 210 C., thereafter precipitating the barium as a water-insoluble barium salt to leave the resultant alpha-amino monocarboxylic acid compound dissolved in the liquor, removing the precipitate and crystallizing the alpha-amino monocarboxylic acid compound from the liquor.

4. In a method of making alpha-amino monocarboxylic acids and salts thereof, the steps or hydrolyzing a hydantoin having the general formula:

a-crr-rm wherein R represent a non-reactive monovalent hydrocarbon radical, by heating, at an autogenous superatmospheric pressure and in the substantial absence of air, one molecular equivalent of the hydantoin together with an aqueous solution of at least 1.5 molecular equivalents of barium a water-insoluble barium salt to leave there- 76 hydroxide to te p ra r s w n 1 5 and 210 6.. thereafter precipitating the barium as a water,- insoluble barium salt to leave the resultant alphaamino monocarboxyllc acid compound dissolved in the liquor, removing the precipitate, and crystallizing the alpha-amino monocarboxyllc acid compound from the liquor.

5. In a. method wherein a hydantoin is hydrolyzed to form a corresponding alpha-amino carboxylate, the steps which consist in effecting the monocarboxylic acid product from the liquor.

6. In a method for making alpha-amin monocarboxylic acids, the steps of hydrolyzing a hydantoin having the general formula:

7. In a. method for making alpha-amino monocarboxylic acids, the steps of Hydrolyzing a. hydantoin having the general formula:

wherein R. represents a non-reactive monovalent hydrocarbon'radical, by heating, at an autogenous superatmospheric pressure and in the substantial absence of air, one molecular equivalent of the hydantoin together with an aqueous solutlon of at least 1.5 molecular equivalents of barium hydroxide to temperatures between 115 and 210 C., thereafter treating the mixture with an agent selected from the group consisting of carbon dioxide and ammonium carbonates in amount sure to temperatures between 115 and 210 C.

9. In a method whereina hydantoin is hydrolyzed to form a corresponding alpha-amino carboxylate, the step of accomplishing the hydrolysis by heating at an autogenous superatmospheric pressure and in the substantial absence of air, one molecular equivalent of the hydantoin together with an aqueous solution of at least 1.5

molecular equivalents of barium hydroxide to temperatures between 115 and 210 C.

4 10. In a method for making alpha-amino monocarboxylic acids, wherein a hydantoin having the 5 general formula:

wherein R represents a non-reactive monovalent hydrocarbon radical, is hydrolyzed by heating the same together with an aqueous barium hydroxide.

solution at an autogenous superatmospheric pressure to temperatures between 115 and 210 0., the steps of subsequently treating the hydrolysis mixture with an agent selected from the group consisting of carbon dioxide and ammonium carbonates in amount sumcient to precipitate the barium as barium carbonate, removing the precipitate and evaporating the liquor to crystallize the alpha-amino monocarboxylic acid product therefrom.

11. In a method for making alpha-amino monocarboxylic acids, wherein a hydantoin, having the general formula:

wherein R and R each represents a member of a group consisting of hydrogen and monovalent organic radicals having a. carbon atom thereof attached to the hydantoin nucleus, is hydrolyzed by heating the same together with an aqueous barium hydroxide solution to temperatures between 115 and 210 C. at an autogenous superatmospheric pressure, and a resultant alphaamino monocarboxylic acid product is separated in an impure crystalline form from the reacted mixture, the step of purifying the crystalline alpha-amino monocarboxylic acid product by washing the same with a liquid monohydric aliphatic alcohol. I

12. In a method for making alpha-amino monocarboxylic acids, wherein a hydantoin having the general formula:

wherein R and R each represents a member of a group consisting of hydrogen and monovalent organic radicals having a carbon. atom thereof attached to the hydantoin nucleus. is hydrolyzed by heating the same together with an aqueous barium hydroxide solution at an autogenous superatmospheric pressure to temperatures between 115 and 210 0.. and a resultant alpha-amino monocarboxylic acid product is separated in an impure crystalline form from the reacted mixture, the step of purifying the crystalline alphaamino monocarboxylio acid product by washing the same with-setbyl alcohol.

13. In a method of making dl-methionine, the step of heating one molecular equivalent of 5-(beta-methylmercapto-ethyl) hydantoin together with an aqueous solution of at least 1.5 molecular equivalents of barium hydroxide at an autogenous superatmospheric pressur to temperatures between 115 and 210 .C.

14. In a method of making dl-methionine. the

steps of heating. at an autogenous superatmoepherlc pressure and in the substantial absence or air, one molecular equivalent of -(beta-methy1- mercaptoethyl) hydantoin together with an aqueous solution of at least 1.5 molecular equivalents of barium hydroxide to temperatures between pressure to temperatures between 115 and 210 C.

16. In a method of making dl-valine, the steps of heating, at an autogenous superatmospheric pressure and in the substantial absence 01' air, one molecular equivalent oi fi-isopropyl hydantoin together with an aqueous solution ot-at least 1.5 molecular equivalents of barium hydroxide to temperatures between 115 and 210 0., there'- after treating the mixture with an agent selected from the group consisting of carbon dioxide and ammonium carbonates in amount sufllcient to precipitate the barium as barium'carbonate, removing the precipitate and crystallizing dl-valine irom the liquor.

17. In a method of making tryptophane, the step or heating one molecular equivalent of 5-(3- indolylmethyl) 'hydantoin together with an aqueous solution of at least 1.5 molecular equivalents oi barium hydroxide at an autogenous superat- 'mospheric pressure to temperatures between 116' and 210 C.

18. In a method of making tryptophane, the steps of heating, at an autogenous superatmospheric pressure and in the substantial absence of air, one molecular equivalent or 5-(3-indolylmethyl) hydantoin together with an aqueous solution of at least 1.5 molecular equivalents of barium hydroxide to temperatures between and 210 C., thereafter precipitating the barium as a water-insoluble barium salt to leave an alkali metal salt 01' the tryptophane product dissolved in the liquor, separating the precipitate and acidifying the liquor to precipitate tryptophane therefrom.

EDGAR. C. BRI'I'ION. JOHN E. LIVAK.

REFERENCES orrnn The following references are or record in the file of this patent: 

1. IN A METHOD WHEREIN A HYDANTION IS HYDROLYZED TO FORM A CORRESPONDING ALPHA-AMINO MONOCARBOXYLATE, THE STEPS WHICH CONSIST IN ACCOMPLISHING THE HYDROLYSIS OF THE HYDANTOIN BY HEATING THE SAME TOGETHER WITH AQUEOUS BARIUM HYDROXIDE AT A SUPERATMOSPHERIC PRESSURE AS GREAT AS THE AUTOGENOUS PRESSURE TO TEMPERATURES ABOVE 115*C., THEREAFTER PRECIPITATING THE BARIUM AS A WATER-INSOLUBLE BARIUM SALT TO LEAVE THE RESULTANT ALPHA-AMINO MONOCARBOXYLIC ACID COMPOUND DISSOLVED IN THE LIQUOR, REMOVING THE PRECIPITATE, CRYSTALLIZING THE ALPHA-AMINO MONOCARBOXYLIC ACID COMPOUND FROM THE LIQUOR, AND WASHING ANY IMPURE CRYSTALLINE ALPHA-AMINO MONOCARBOXYLIC ACID PRODUCT WITH A LIQUID MONOHYDRIC ALIPHATIC ALCOHOL TO REMOVE THE IMPURITIES.
 18. IN A METHOD OF MAKING TRYPTOPHANE, THE STEPS OF HEATING, AT AN AUTOGENOUS SUPERATMOSPHERIC PRESSURE AND IN THE SUBSTANTIAL ABSENCE OF AIR, ONE MOLECULAR EQUIVALENT OF 5-(3-INDOLYLMETHYL) HYDANBTOIN TOGETHER WITH AN AQUEOUS SOLUTION OF AT LEAST 1.5 MOLECULAR EQUIVALENTS OF BARIUM HYDROXIDE TO TEMPERATURES BETWEEN 115* AND 210*C., THEREAFTER PRECIPITATING THE BARIUM AS A WATER-INSOULBLE BARIUM SALT TO LEAVE AN ALKALI METAL SALT OF THE TRYPTOPHANE PRODUCT DISSOLVED IN THE LIQUOR, SEPARATING THE PRECIPITATE AND ACIDIFYING THE LIQUOR TO PRECIPITATE TRYPTOPHANE THEREFROM. 